This invention relates to the ability to treat naturally occurring clay deposits and more particularly, to selectively disperse discrete, individual particles of Attapulgite while liberating other non-Attapulgite minerals such as Montmorillonite, Sepiolite, Bentonite, Calcium Carbonate, Silica and Kaolin, from in between the Attapulgite Bundles. The invention relates to a dry clay which is readily re-dispersed.
Attapulgite Clay particles naturally occur as colloidal, high aspect ratio, rod shaped particles that are tightly bundled together as clusters. The rods tend to be all equal in particle length and diameter, similar to a group of bound pipes or drinking straws. The individual clusters are agglomerated randomly. The Attapulgite deposits located in northern Florida and southeastern Georgia were formed in shallow, magnesium rich bay waters, where other clay minerals were formed simultaneously or were introduced through air and water movements. Other minerals such as silica, calcium carbonate and magnesium carbonate were also formed in place or were transported into the deposit during its formation, resulting in a variety of non Attapulgite materials being present in levels approaching 20 weight percent or more. In order for the Attapulgite to be in a physical form acceptable for commercial use, it needs to be processed into a powder. The Attapulgite clay is historically processed by selectively mining the deposit, classifying the ore according to its grit content and gelling properties, drying, mechanically grinding or milling ore with minimal amounts of contaminants, followed by particle size separation and drying to a moisture content of approximately 10% to 6%. The non-Attapulgite matter is also reduced in size and is partially removed during the particle size classification process. The non-clay minerals can be abrasive and must either be removed or reduced in size to lessen their abrasive characteristics in order for the Attapulgite powder to be useful in end use applications.
A dry grinding/milling operation will partially break up the bundle clusters with the undesirable result of fracturing of individual Attapulgite particles. This may be, in part, due to the presence of grit and particles of non-clay matter which are present in the clay ore and which break and damage the individual rods during the dry grinding/milling operation. The individual Attapulgite particles provide the absorptive, thixotropic, anti-settling, and/or binding properties to a wide variety of end use applications. The crystal structure of Attapulgite clay has positive and negatively charged sites on the lateral surfaces of each particle and at the ends of each individual particle. When the crystals were formed they joined together in the straw clusters with random bundles orienting themselves to neutralize the charges. When these individual particles are dispersed by chemical or mechanical means the particles seek to satisfy the negative and positive charges by randomly re-connecting particle to particle instead of forming bundles and clusters. This unique feature of attapulgite clay creates the thixotropic properties and binding properties of Attapulgite. Particles with higher length to width, or aspect ratios, typically are more efficient than particles with lower aspect ratios in providing products with higher gel strength and binding properties.
In end use applications, a chemical dispersant, predominantly tetrasodium pyrophosphate (TSPP), will at best, separate a majority of the Attapulgite bundles into individual particles in an aqueous medium under moderate-to-high shear agitation. TSPP will also disperse other clay types present. Individual clay particles of each clay species present, will remain suspended in water. U.S. Pat. No. 3,569,760 has shown that non-clay minerals will not remain suspended in the water and will settle to the bottom due to gravitational forces if the clay-water slurry is low enough in viscosity. The relatively large non-clay minerals, which by their means of higher hardness, can resist size reduction and thus can also be removed by screening, centrifuging, via sedimentation tanks, hydrocyclone, or other physical separation means. The non-Attapulgite clay minerals, such as Montmorillonite, Smectite, and Sepiolite, will remain mixed in with the Attapulgite clay. The non-Attapulgite clays do not have the same performance characteristics as Attapulgite clay, some which are detrimental to performance in particular applications. At the present time, the suppliers and users of Attapulgite are dependent on the quality of natural deposits for the concentration of non-Attapulgite clay content. The availability of high purity Attapulgite clay relies on the economically inefficient selective mining of the deposits.
Gantt et. al. in U.S. Pat. No. 5,358,120, have shown that other types of previously commercially available dispersants such as sodium polyacrylates can be used with clay. However, sodium polyacrylates with molecular weights less than 4,300, while effective on kaolins and bentonites, are typically not as effective with Attapulgite when compared to TSPP, and thus are not generally used.
Attapulgite clay provides thixotropic and binding properties through the process of re-flocculating after being typically dispersed at less than 5% solids in an aqueous system, whereby a gel structure is generated. Various salts, at additive levels, can act as re-flocculating agents.
Historically, Attapulgite clay is dried to a free moisture content of approximately, 10-16%, with an 8 or 9% minimum. At lower moisture contents, the Attapulgite begins to lose its thixotropic properties and does not readily re-disperse in water. Brooks, in U.S. Pat. No. 4,966,871, has shown that it is possible to vacuum dry Attapulgite down to less than 2% free moisture and still retain its thixotropic properties, the importance being that the attapulgite is dried under vacuum conditions.
Attapulgite clay is frequently used as an additive product, often comprising only 1 to 3 weight percent of the final composition. Attapulgite clay does not disperse completely in a liquid medium at these low concentrations. The believed cause is the lack of clay to clay collisions needed to break up the agglomerates and bundles. A technique used to overcome this behavior is one in which a 20 to 25% slurry is made, and when completely dispersed, it is diluted with water down to the desired ultimate concentration.
There is a need to be able to efficiently and economically separate Attapulgite clay from other clay types, from non-clay minerals while not destroying the high aspect ratio of the particles with mechanical grinding force. There is a need to reduce the moisture content of attapulgite while maintaining gel properties to enhance it economic benefit through concentration of the product for its end use application.
It is an object of the invention to provide an economical and efficient method to separate Attapulgite clay from other clay components and minerals in clay ore.
It is an object of the invention to use the method to allow for the mining of lower grade deposits which have historically not been useable due to high concentrations of non-clay minerals.
It is an objective of the invention to use the method for the blending of attapulgite ores in slurry form to efficiently produce products with uniform physical and chemical properties.
It is a further object of the invention to provide a purified Attapulgite clay.
It is yet another object of the invention to provide dry Attapulgite clay particles which are readily dispersed in water without addition of dispersant or surfactant.
It is a further object of the present invention to provide a dry particulate Attapulgite clay which has less than 0.5% free moisture content and retains thixotropic properties without using vacuum drying.
In accordance with the teachings of the present invention, there is disclosed a method of processing naturally occurring clay ore to separate Attapulgite clay from other materials. This includes the steps of crushing the clay ore, adding sodium polyacrylate to water, the sodium polyacrylate having a molecular weight between 4,000 and 5,000, preparing a slurry of the clay ore with the aqueous sodium polyacrylate to disperse the Attapulgite clay in the water, dispersing the Attapulgite without fully dispersing the non-Attapulgite Minerals, separating the non-Attapulgite minerals from the Attapulgite and drying the dispersed, purified Attapulgite clay.
Additionally, there is disclosed a wet method of processing naturally occurring clay ore to separate Attapulgite clay from other materials. The method has the steps of crushing the clay ore and adding a dispersant to water. A slurry of the clay ore is prepared with the aqueous dispersant to disperse the Attapulgite clay in the water. The dispersed Attapulgite clay is separated from the non-Attapulgite materials, and the dispersed clay is dried to a free moisture content of approximately 2-3%.
There is further disclosed a wet method for separating and dispersing Attapulgite clay. Naturally occurring clay is crushed. A dispersant is added to water. A slurry of the naturally occurring clay is prepared with the aqueous dispersant wherein the clay is dispersed in the water. Non-Attapulgite materials are separated from the aqueous slurry of Attapulgite clay. The dispersed Attapulgite clay is dried. Up to approximately 35% by weight of the dry Attapulgite clay is added to water. The Attapulgite clay is re-dispersed in the water using a low shear mixer without the addition of further dispersant or a surfactant.
Still further, there is disclosed a purified dried, particulate Attapulgite clay comprising Attapulgite clay separated from non-clay matter, the Attapulgite clay having a free moisture content of approximately 2-3%.
These and other objects of the present invention will become apparent from a reading of the following specification, taken in conjunction with the enclosed drawings.